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France has been alarmed by a Russian-backed offensive against rebel-held parts of Aleppo

The besieged east of Aleppo has come under intense aerial bombardment since a cessation of hostilities brokered by the US and Moscow collapsed last month.

The area was hit again on Tuesday in some of the heaviest air strikes in days, a monitoring group and activists said.

At least 12 civilians were killed in the strikes and considerable damage was inflicted on several areas, according to the UK-based Syrian Observatory for Human Rights.

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Media captionAleppo: Key battleground in Syria's civil war

The observatory said Russian war planes had dropped so-called "bunker busters" and other bombs, with children thought to be among the dead.

Some of the most intense attacks have come in the Bustan al Qasr neighbourhood near the citadel, which sits near the front line with the government-held west.

The strikes followed a temporary lull in the bombardment called by the Syrian government, partly to allow civilians to leave opposition-held eastern areas of the city.

Aid organisation Medecins Sans Frontieres said on Tuesday there were now only 11 working ambulances left in the city because five had been hit by bombs and another eight were missing essential spare parts.

Diplomatic efforts to revive the ceasefire have so far come to nothing.

The UN has warned that eastern Aleppo, where an estimated 275,000 people still live, could face "total destruction" in the space of two months.

Last week, Russia vetoed a UN Security Council resolution drafted by France calling for an end to the bombing in Aleppo.